Comparison of cornea and skin multiple pulse injury thresholds with laser MPEs

According to current ANSI Z136.1, IEC 60825-1 and ICNIRP guidelines, two exposure limit criteria apply for the cornea and skin: the single pulse limit and the average irradiance limit. The reduction factor C_P for repetitively pulsed exposures need only be applied to retinal thermal limits, not to limits to protect the skin and cornea from thermally induced injury. Since only a very limited amount of animal studies for multiple-pulse thresholds are available for the cornea and skin, we have used a computer model to systematically study the threshold trends for exposure to multiple pulses. For a number of representative wavelengths and irradiance diameters, and two pulse durations, injury thresholds as predicted by computer models were compared to the two exposure limit criteria as a function of duty cycle (i.e. repetition rate), number of pulses and as a function of exposure duration. The results support the current multiple-pulse criteria for the cornea and the skin, i.e. no additional reduction of the single pulse limit by C_P: for those repetition rates where the single pulse limit is the limiting criterion, the reduction of injury threshold compared to the single pulse is weak. For higher repetition rates, when the average irradiance limit is the limiting criterion the worst case is a cw exposure (duty cycle 100%) and reducing the duty cycle, i.e. pulsed exposure, leads to increased (less critical) thresholds.